This invention relates to an optical fiber connector for communication purpose, and more particularly to an optical fiber connector which can easily and precisely correct any disagreement in the optical axes of optical fiber cores respectively inserted into a pair of ferrules.
An optical fiber consists of a cylindrical core forming a path for light and a cladding fittingly and concentrically surrounding the outer periphery of the core. The core and the cladding are made of glass of different refractive index.
When an optical signal is connected with the core, the signal is reflected by the cladding due to the light refractive index difference from that of the core. Thus, the brightness of the light is not attenuated by outward diffusion, and the optical signal may be transmitted for a long distance. However, the length of a fiber is limited when the fiber is manufactured, and so many connecting portions arise even for a single circuit.
For connecting the optical fibers, the end surfaces of the fibers may be heated to a very high temperature and welded with each other, or an optical fiber connector may be used.
FIG. 1 is a sectional view showing an example of a conventional optical fiber connector. In the figure, the reference numerals 1, 1a denote optical fibers, 2, 2a denote ferrules and 3 denotes a sleeve. In the conventional optical fiber connector that is shown, a pair of optical fibers 1 and 1a to be connected with each other are beforehand bonded and fixed to the respective holes bored at the centers of ferrules 2, 2a. The end surfaces 4, 4a of the ferrules 2, 2a are polished into mirror faces and then they are forcibly inserted into the sleeve 3, so that the optical axes of the two facing optical fibers 1 and 1a precisely agree with each other.
In order to avoid any attenuation of brightness of the optical signal, the diameter of the core of the optical fiber 1 or 1a is extremely minute in size. For example, in the case of a single mode type optical fiber for long-distance use, it may be only 10 microns. Even for a multimode type optical fiber for middle- and short-distance use, 50 micron fiber has generally been used. The diameter of the cladding in either case is 125 microns.
Even with the connecting method using an optical fiber connector, loss of light at the connection cannot be avoided. The largest cause for this loss at the connection is fiber misalignment, or the positional disagreement between the centers of the pair of optical fiber cores.
FIG. 2 is an enlarged view of a section of the conventional optical fiber connector shown in FIG. 1, cut transverse to the axis of the connector of FIG. 1.
In FIG. 2, the core and the cladding of the optical fiber 1 are resepectively given by 1A and 1B. The cladding 1B is made integral with the optical fiber core 1A. When a pair of fibers 1, 1a are connected, the respective optical fibers 1, 1a are inserted into holes 5 bored in ferrules 2 and bonded thereto respectively and then they are forcibly inserted into the sleeve 3.
Causes for the positional disagreement between the centers of the optical fiber cores may be listed as follows:
(A) Causes from optical fibers
.circle.1 Disagreement between the center of the core and the circumference of the cladding . . . 1 micron; and PA1 .circle.2 Disagreement caused by the error in size of the outer diameter of the cladding . . . 1 micron. PA1 .circle.1 Disagreement caused by an error in the size of the hole of the ferrule . . . 1 micron; PA1 .circle.2 Disagreement between the center of the hole of the ferrule and the outer diameter . . . 2 microns; and PA1 .circle.3 Disagreement between the diameters of a pair of ferrules . . . 1 micron.
(B) Causes from ferrules
There has been a possibility of disagreement of five microns in a single ferrule, and 11 microns at the maximum when a pair of ferrules are inserted into a sleeve. In the case of an optical fiber core of 10 microns, for example, the two cores would be completely displaced. And in the case of a 50 micron fiber core, the light transmitting surfaces would overlap by only 70% and the loss at the connection would reach about 2 dB, so that the desirable value of 1 dB could not be attained.
Various proposals have been made for avoiding the disagreement between the centers of the optical fiber cores.
One of such proposals is to raise the accuracy in the size of the ferrule itself. However, improving the accuracy in size would be extremely expensive, and also mass-production of such good ferrules would be difficult. Furthermore, any error inherent in the optical fibers themselves could not be curred.
One method has been proposed for solving this problem. According to it, after an optical fiber has been inserted into a ferrule, the loss at the connection is measured while the fibers are connected. Then the ferrule is rotated to find out the position of the lowest connection loss and then the position is locked, for example, with a positioning key, etc. Thus the relative phase relation between the pair of ferrules are determined. However, in view of the high precision required in manufacturing an optical fiber connector of very minute size, few grooves may at most be formed in the positioning key. It has not been easy to provide continuous adjustment with the positioning key, and precise correction could thus not be expected.